Methods for wafer level burn-in

ABSTRACT

A method, circuit and system for determining burn-in reliability from wafer level burn-in are disclosed. The method includes recording the number of failures in each IC die in nonvolatile elements on-chip at points in time over the duration of wafer level burn-in testing. Methods may also include using a supervoltage level to signal a transition between cycles of burn-in testing. The number of failures in each IC die, along with their associated points in time, may be used to create burn-in reliability curves which are conventionally derived using other processes that may be less cost effective or not possible to effect with unpackaged IC dice. Unused registers of nonvolatile elements may also be determined by reading the nonvolatile element registers on a semiconductor die on the wafer. Circuits and systems associated with the method of the present invention are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 11/065,016,filed Feb. 24, 2005, pending, which is a divisional of U.S. patentapplication Ser. No. 10/207,526, filed Jul. 29, 2002, now U.S. Pat. No.6,943,575, issued Sep. 13, 2005, which application is related to U.S.patent application Ser. No. 08/946,027, filed Oct. 7, 1997, now U.S.Pat. No. 6,190,972, issued Feb. 20, 2001, and U.S. patent applicationSer. No. 09/651,858, filed Aug. 30, 2000, now U.S. Pat. No. 6,829,737,issued Dec. 7, 2004.

TECHNICAL FIELD

This invention relates generally to integrated circuit (IC) testcircuits and methods. In particular, the present invention relates tostorage of wafer level burn-in test data on the IC device under testand, more particularly, the storage and retrieval of wafer level burn-indata to and from nonvolatile memory elements in the IC device forgenerating burn-in reliability curves.

BACKGROUND OF THE INVENTION

Semiconductor IC devices are manufactured on wafers or other bulksubstrates of semiconductor material. Conventionally, many devices aremanufactured on a single wafer and individual devices or groups ofdevices are singulated from the wafer and packaged. The IC devices aretested at various points during the manufacturing process, i.e., withelectrical probes while they are still on the wafer and then afterpackaging. The terms “IC,” “device” and “IC device” are usedinterchangeably herein.

During testing, a particular signal or combination of signals is inputto the IC device and the output value or values read from the device arecompared with expected values from a properly functioning device. Testsmay involve a particular signal or combination of signals beingdelivered repetitively, perhaps under extreme environmental conditions(temperature, voltage, etc.) in order to identify a device which wouldfail after a shorter than usual period of use (“burn-in” testing). Othertests may involve a number of different signals or signal combinationsdelivered in sequence. One method for testing a memory device is todeliver the same signal or signal combination to multiple identicalsubsections of a memory array in the memory device simultaneously andcompare the values read from the subsections (“compression testing”). Ifall of the respective, read values match, the test has been passed,while a mismatch between respective values read from any of thesubsections indicates a memory device malfunction and failure of thetest.

A particular test or test sequence often includes multiple test steps.Moreover, a complete test flow will often require that IC devices movefrom one piece of test equipment to another. For example, a first pieceof test equipment and test fixtures may be utilized for probe testing,another for burn-in testing and yet another for packaged device testingafter burn-in.

After a particular test or test sequence has been completed, IC devicesthat have failed some portion, or all, of a test may be separated fromthe good devices. However, an IC device that has failed one portion ofthe test sequence may pass subsequent test sequences. So, if the failingIC device is erroneously placed into the “good” bin and then passessubsequent tests, it may eventually be classified and sold as fullyfunctional. One way to avoid this type of error is to store informationregarding the test history of the device on the device itself innonvolatile memory elements. One example describing storage of testresults in nonvolatile memory on a semiconductor device is co-pendingU.S. Pat. No. 6,190,972, issued Feb. 20, 2001, the disclosure of whichis hereby incorporated herein by reference for all purposes. A methodand system of storing device test information on a semiconductor deviceusing on-device logic for determination of test results are disclosed inco-pending U.S. Pat. No. 6,829,737, issued Dec. 7, 2004, the disclosureof which is hereby incorporated herein by reference for all purposes.

Defects in a finished IC chip assembly can prevent it from operating asintended. In spite of painstaking attention to detail, defects may beintroduced at various levels of production. For example, manufacturingdefects in the IC die may cause a failure. It has been found, however,that some defects manifest themselves immediately, while other defectsmanifest themselves only after the IC die has been operated for someperiod of time.

“Burn-in” refers to the process of accelerating failures that occurduring the infant mortality phase of component life in order to removethe inherently weaker ICs. Burn-in testing has been regarded as acritical process for assuring product reliability since thesemiconductor industry began. There are various types of conventionalburn-in testing. During a process known as “static” burn-in (alsoreferred to as “dumb” burn-in testing), temperatures may be increased(or sometimes decreased) while the pins are biased but not toggled. Nodata is written to the IC, nor is the IC exercised under stress duringstatic burn-in. During “unmonitored dynamic” burn-in testing,temperatures may be increased while the pins on the test IC are toggled.For a memory IC undergoing unmonitored dynamic burn-in testing, datapatterns are written to the memory IC, but not read, while being cycledunder stress. Hence, with unmonitored dynamic burn-in testing, there isno way of knowing whether the data written is retained by a memory cell.

In recent years, as memory systems have grown in complexity, the needfor more reliable components has escalated. More sophisticated methodsof screening infant mortalities have been developed. As IC manufacturingpractices have become more consistent, it has become clear that burn-insystems that simply provide stress stimuli in the form of hightemperature and VCC (power) to the IC under test may be inadequatebecause such burn-in systems cannot detect and screen infant mortalityfailure rates measured in small fractions of a percent.

To address these issues, an “intelligent” burn-in (sometimes referred toas “smart” burn-in) testing can be utilized. The term “intelligentburn-in,” as used in this discussion, refers to the ability to combinefunctional, programmable testing with the traditional burn-in cycling ofan IC under test in the same chamber. Advantages to this approachinclude: (1) the ability to identify when a failure occurs and, thereby,compute infant mortality rates as a function of burn-in time (and as aresult, an optimal burn-in time for each product family may beestablished); (2) the ability to correlate burn-in failure rates withlife test data typically obtained by IC manufacturers to determine thefield failure rates of their products; and (3) the ability toincorporate into the burn-in process certain tests traditionallyperformed using automatic test equipment (ATE) systems, thereby reducingcosts.

Reliability curves are used to express an instantaneous failure ratef(t) over time t, and often have a “bathtub” shape. The reliabilitycurves for many, if not all, ICs are generally like that shown inFIG. 1. The reliability curve in FIG. 1 may be divided into threeregions: (1) an infant mortality region, (2) a random failures region,and (3) a wearout region.

The infant mortality region begins at time to, which may occur uponcompletion of the manufacturing process and initial electrical test.Some ICs, of course, fail the initial electrical test. Inherentmanufacturing defects are generally expected in a small percentage ofICs, even though the ICs are functional at time to. Because of theseinherent manufacturing defects (that may be caused by contaminationand/or process variability), these ICs have shorter lifetimes than theremaining population. While ICs with failures occurring in the infantmortality region may constitute a small fraction of the totalpopulation, they are the largest contributor to early-life failurerates.

Once ICs subject to infant mortality failure rates have been removedfrom the IC population, the remaining ICs have a very low and stablefield failure rate. The relatively flat, bottom portion of the bathtubcurve (FIG. 1), referred to as the random failure region, representsstable field-failure rates which occur after the IC failures due toinfant mortalities have been removed and before IC wearout occurs.Eventually, as wearout occurs, the failure rate f(t) of the ICs beginsto increase rapidly. However, the average lifetime of an IC is nottypically well known, because most laboratory tests simulate only a fewyears of normal IC operation.

FIG. 2 illustrates reliability curves measured for exemplary IC devicelots A, B and C. FIG. 2 only includes the front end of the bathtub curveillustrated in FIG. 1 for each device lot. Lot A is characterized by ahigh instantaneous failure rate f(t) that does not improve after anylength of time. Lot B is characterized by a stable failure rate that hasimproved after the infant mortality region, but remains above a selectedmanufacturing process standard (dotted line). If devices must meet theselected manufacturing process standard, then lots A and B must bescrapped because the stable failure rates obtained in the randomfailures region is too high. Lot C is characterized by a stable failurerate that is within the selected manufacturing process standard and,thus, may be considered a good lot and suitable for sale. Burn-intesting provides data for determining instantaneous failure rate curves.Determining the instantaneous failure rate curves serves at least twouseful functions: (1) one can determine when burn-in testing iscomplete, i.e., how long burn-in testing must be performed to weed outthe infant mortality failures, and (2) one can determine if burn-intesting will complete, i.e., occasionally lots like A or B illustratedin FIG. 2 occur and must be scrapped because they never clean up.

There are at least four approaches to ensuring IC reliability: (1)evaluation of data on a per-die basis by burn-in stressing at theindividual IC die level, (2) evaluation of data on a per-die basis byburn-in stressing at the packaged component level, (3) evaluation ofdata on a per-wafer basis by burn-in stressing at the wafer level, and(4) evaluation of data on a per-lot basis by analyzing a sample ofwafers per lot. The first of these approaches may be impractical if theIC die has failures and before redundancy has been enabled. The secondapproach is quite common in the industry, but does not allow foridentification of infant mortalities until all of the packaging stepshave been completed. The fourth approach is unacceptable from areliability standpoint since any single wafer within a lot may haveunique reliability problems.

Thus, there exists a need in the art for a system, circuit and methodfor determining wafer level burn-in reliability.

SUMMARY OF THE INVENTION

The present invention includes a method, circuit and system fordetermining burn-in reliability from wafer level burn-in. The methodaccording to the present invention includes recording the number offailures in each IC die in nonvolatile elements on-chip at points intime over the duration of wafer level burn-in testing. The number offailures in each IC die along with their associated points in time maybe used to create burn-in reliability curves which are conventionallyderived using other processes that may be less cost effective. Circuitsand systems associated with the method of the present invention are alsodisclosed.

Advantages of the method, circuit and system of the present inventioninclude being able to determine burn-in reliability at the wafer level,determining whether all infant mortalities have been identified,determining whether additional wafer level burn-in is necessary or ifpackaged part burn-in is necessary, determining whether there is a badwafer in a good lot, or if an entire lot is bad that needs to bescrapped at the wafer level and before packaging and the ability toidentify known good dice suitable for higher-order subassemblies or forwafer sales or die sales once singulated.

The embodiments of the present invention will be readily understood byreading the following detailed description in conjunction with theaccompanying figures of the drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The drawings illustrate exemplary embodiments of the present invention,wherein like reference numerals refer to like parts in different viewsor embodiments in the drawings.

FIG. 1 is a graphical representation of a reliability curve thatexpresses an instantaneous failure rate r(t) as a function of time, t.

FIG. 2 illustrates reliability curves measured for integrated circuit(IC) device lots A, B and C.

FIG. 3 is a graph of exemplary wafer level burn-in data stored innonvolatile elements on each IC die during each cycle of burn-in testingaccording to the present invention.

FIG. 4 illustrates a bank of antifuse registers in accordance with thepresent invention.

FIG. 5 is a flow chart illustrating a method for determining burn-inreliability from wafer level burn-in of the present invention.

FIG. 6 is a block diagram of a memory device including wafer levelburn-in circuitry of the present invention.

FIG. 7 is a plan view of a semiconductor substrate including at leastone IC die having wafer level burn-in circuitry of the presentinvention.

FIG. 8 is a block diagram of a system for performing wafer level burn-incircuitry of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention includes a method, circuit and system fordetermining burn-in reliability from wafer level burn-in. Conventionalburn-in reliability curves are generated by burn-in testing packagedparts for failures and making determinations whether to accept or rejectthe lot associated with the packaged parts. The invention disclosedherein provides the capability of generating burn-in reliability curvesby performing wafer level burn-in and measuring defects at the IC dielevel before the IC die has been packaged. According to the presentinvention, wafer level burn-in data may be stored in and retrieved fromon-chip nonvolatile elements. The present invention facilitates the saleof burned-in dice either in die form or wafer form. For example,wafer-level burned-in dice may be used in multi-chip modules (MCMs) andother kinds of semiconductor assemblies. The invention may also providean early measure of burn-in reliability and provides process engineerswith information that may be used to accept or reject a wafer or lotassociated with the IC die subject to wafer level burn-in testing.

Once a bulk substrate has been processed to form a plurality of IC dicethereon, a sacrificial layer of metal may be applied to the bulksubstrate to deliver power VCC, ground GND and any other signal(s)necessary to perform burn-in testing to each IC die. According to thepresent invention, another signal may be a supply voltage, CMN, forprogramming nonvolatile elements such as antifuses. A supply voltage,CMN, may be referred to herein as an “antifuse supply CMN.” Asupervoltage level on the antifuse supply CMN may be used to signal atransition between cycles of burn-in testing as described below. Asknown to one of ordinary skill in the art, nonvolatile elements may beprogrammed using voltages substantially above (supervoltage) the nominalvoltage of the power supply, VCC, depending on the technology used toimplement the nonvolatile storage. The sacrificial metal layer may beused to perform the wafer level burn-in according to the presentinvention. After the wafer level burn-in is complete, the sacrificialmetal layer may be removed. The application and removal of metal layersare within the knowledge of one of ordinary skill in the art and, thus,will not be further elaborated upon herein. Of course, one of ordinaryskill in the art will recognize that other semiconductor technologiesmay employ different voltage supplies and methods of delivering power toindividual IC dice consistent with the present invention.

FIG. 3 is a graph 300 of cumulative failures detected on an IC dieduring wafer level burn-in testing according to the present invention.As shown in FIG. 3, there may be four quarters of burn-in testing, eachseparated by time stamps, t₀-t₄. The number of quarters or cycles ofburn-in testing is not critical to the invention. Four quarters ofburn-in are illustrated only for convenience of discussion. Each quarterof burn-in testing may include “stressing” followed by “testing” todetermine the number of failures. “Stressing” may be accomplished by aburn-in self-stress (BISS) mode performed, for example, by dedicatedlogic on each individual IC die. BISS may include exercising the part bywriting and reading to locations in the memory array, for example.“Testing” may be accomplished by a burn-in self-test (BIST) modeperformed, for example, by dedicated logic on each individual IC die.BIST may include writing to memory cell locations, reading the contentsof the cells written to and comparing the data written against the dataread as known to one of ordinary skill in the art. The BISS and BISTcircuitry may be integrated as a single functional logic block with BISSand BIST test modes.

According to the present invention, the number of device failures may bedetected and stored in nonvolatile elements at each time stamp, t₀-t₄.As noted above, the number of failures may be detected using an on-chipBIST. That same BIST may be used to cycle or stress the IC die duringeach of the four quarters of burn-in testing, for example, using abuilt-in self-stress (or self-burn) test mode of the BIST circuitry. Thephysical implementation of a “time stamp” according to the presentinvention may be a single bit of an n-bit register or the n-bit registeritself. For example, the single bit may be used to indicate when aquarter or cycle has been or has not been completed. On-chip circuitrymay be used to look at the “time stamp bit” to see if its associatedregister has been used. On-chip circuitry may then look for the nextregister with an intact time stamp bit in a predefined order for storageof new data.

Wafer level burn-in testing according to the present invention may beginwith detecting the number of failures at time stamp to, prior to thefirst quarter of burn-in stressing. Detecting the number of failures attime stamp to may be performed using BIST circuitry. The number offailures detected on the IC die and the time stamp may be recorded orstored in nonvolatile elements, such as antifuse registers, on the ICdie itself. Then, the first quarter of burn-in stressing may beperformed. At the end of the first quarter, t₁, the number of failureson the IC die is again detected (using BIST, for example) and stored innonvolatile elements along with the time stamp, t₁. This procedure maybe repeated for all four quarters of the wafer level burn-in testing, atwhich point the nonvolatile elements on each IC die contain wafer levelburn-in testing data that may be used to generate burn-in reliabilitycurves (bathtub curves). The nonvolatile elements may be fuses,antifuses or any other suitable nonvolatile storage elements.

FIG. 4 illustrates a bank 400 of nonvolatile registers suitable for useas the nonvolatile elements as described above. As shown in FIG. 4, thebank 400 may include five 8- registers, one for each time stamp, t₀-t₄.Each 8-register may include one bit for a time stamp and seven bits forstoring the number of failures detected at the associated time stamp.The time stamp bit may be used, for example, by a state machine orincremental pointer that increments a pointer to the registers duringthe cycles of the burn-in testing so that the appropriate information isstored. The number of bits per register is not critical to theinvention, as more or fewer bits may be used depending on the size ofthe memory array being tested or the quality of the manufacturingprocess. Generally, there may be m+1 registers, each having n bits,where m and n are positive integers and m represents the number ofcycles of burn-in testing. The physical arrangement of the bank 400 ofnonvolatile registers and methods for storing information in andretrieving information from nonvolatile registers are within theknowledge of one of ordinary skill in the art and, thus, will not befurther elaborated herein.

FIG. 5 is a flow chart illustrating a method 500 for determining burn-inreliability from wafer level burn-in according to the present invention.Once a wafer has been manufactured and a sacrificial layer of metal hasbeen placed on the wafer to provide power to the IC dice during waferlevel burn-in, method 500 may include detecting 502 a signal indicatinga transition between one of a number m of cycles of burn-in testing.According to one embodiment of the present invention, the number m ofcycles of burn-in testing may be four quarters of burn-in testing, i.e.,m=4.

During each quarter of burn-in testing, voltage levels may be elevatedto greater or lesser degrees relative to nominal values in order tostress the circuitry and thereby quickly identify “infant mortality”failures. For example, if the nominal operating voltage VCC=2.5 volts,the power supply VCC may be elevated from nominal to approximately 3.4volts during first and second quarters of burn-in. Using the sameexample (i.e., nominal operating voltage VCC=2.5 volts), the powersupply VCC may be elevated from nominal to approximately 3.2 voltsduring third and fourth quarters of burn-in. Where the nominal operatingvoltage VCC=3.3 volts, the power supply VCC may be elevated from nominalto approximately 4.5 volts during first and second quarters of burn-in.Using the same example (i.e., nominal operating voltage VCC=3.3 volts),the power supply VCC may be elevated from nominal to approximately 4.2volts during third and fourth quarters of burn-in.

Additionally, each cycle of burn-in testing may encompass a predefinedduration, e.g., 4 to 6 hours. Various approaches to burn-in stressingICs are within the knowledge of one of ordinary skill in the art and,thus, will not be further elaborated herein.

Any suitable signal may be used to indicate a transition between thenumber m of cycles of burn-in testing. According to an embodiment of thepresent invention, a nonvolatile supply voltage, e.g., antifuse supply(CMN) may be used to indicate a transition between cycles of burn-intesting. More specifically, a rising edge on a “supervoltage” associatedwith an antifuse supply CMN may be used to indicate a transition betweencycles of burn-in testing. Detecting a supervoltage may be accomplished,for example, with high voltage detect circuitry (see FIG. 6 and relateddiscussion below).

Method 500 may further include recording 504 a time stamp associatedwith the transition between the cycles of wafer level burn-in. Thedetection 502 of a signal indicating a transition between one of anumber m of cycles of burn-in testing marks a transition between abuilt-in self-stress (BISS) mode and a built-in self-test (BIST) mode. ABIST is used to detect the number of failures that occurred as a resultof the BISS to date, see below. As described above, the time stamps,t₀-t₄, may be a single bit of a register associated with storing thenumber of failures detected to that point in time. Additionally, thetime stamps, t₀-t₄, may be used by a state machine or pointer duringwafer level burn-in to cycle through the antifuse registers (i.e.,nonvolatile elements).

Method 500 may further include performing 506 a BISS/BIST to determine acurrent number of failures associated with the current cycle asindicated by the time stamp. According to the invention, a BISS/BIST maybe performed to identify the current number of failures in the IC die.Performing 506 the BISS/BIST may include compressing addresses from amemory array into redundancy space and reading errors corrected inredundancy space. Recording 504 a time stamp may be performed before orafter performing 506 a BISS/BIST.

Method 500 may further include storing 508 the current number offailures associated with the particular time stamp, t₀-t₄. Storing 508may include programming nonvolatile elements on the IC die. As describedabove, nonvolatile elements may be fuses, antifuses or any othernonvolatile storage elements in accordance with the present invention.As described above, the nonvolatile elements may be a bank 400 ofantifuse registers, see FIG. 4 and related discussion.

According to method 500, the above steps may be repeated 510 for eachnew cycle of burn-in testing, e.g., each quarter of burn-in testing. Thenumber of quarters or cycles of burn-in testing is not critical to theinvention. The four quarters of burn-in testing described herein aremerely exemplary. After the wafer level burn-in is complete (e.g., allfour quarters of burn-in testing are complete), the sacrificial metallayer is removed and the wafer is sent to wafer probe.

Method 500 may further include reading 512 the time stamp and number offailures associated with the time stamp for each of the time stamps,i.e., the wafer level burn-in data. Reading 512 of the time stamp andcurrent number of failures associated with the time stamp (i.e., waferlevel burn-in data) is performed by wafer probe equipment. Method 500may further include generating 514 wafer level burn-in reliabilitycurves to determine if the wafer has acceptable failure rates.Additionally, method 500 may further include selling 516 acceptablewafers. The wafer level burn-in data may be used to generate burn-inreliability curves, i.e., bathtub curves, which in turn give a processengineer information about whether the IC dice, wafer and lot areacceptable or if there is a problem that needs to be addressed.

FIG. 6 is a block diagram of a memory device 600 suitable forfabrication as a semiconductor die including wafer level burn-incircuitry 650 of the present invention. Memory device 600 may furtherinclude built-in self-stress (BISS) and built-in self-test (BIST)circuitry 614 and a memory array 612, both of which are in communicationwith the wafer level burn-in circuitry. Wafer level burn-in circuitry650 may include nonvolatile elements 602 configured for storing defectinformation detected during each stage of burn-in testing. Nonvolatileelements may be fuses, antifuses or any other suitable nonvolatile datastorage element. In an embodiment of the present invention, nonvolatileelements may form a bank of antifuse registers. As noted above, the bankof antifuse registers may include five 8-antifuse registers. Eachantifuse register may have a time stamp bit, seven bits for storing thenumber of failures detected. The nonvolatile elements 602 are formed onthe die itself.

Wafer level burn-in circuitry 650 may also include signal detectioncircuitry 604 for detecting a signal indicating initiation of a cycle ofwafer level burn-in testing. Signal detection circuitry 604 may be highvoltage detect circuitry suitable for detecting the initiation of ashort supervoltage pulse on an antifuse supply CMN. Of course, one ofordinary skill in the art will recognize that there are many signaldetection circuits that are suitable for the purposes of the presentinvention. All such suitable signal detection circuits are considered tobe within the scope of the present invention.

Wafer level burn-in circuitry 650 may also include address compressioncircuitry 606 which may be coupled to the memory array 612 and the BISSand BIST circuitry 614 for compressing cells of a memory array intoredundancy space. Repairable failures can have variations in bit counts.Most memory IC dice are not prime (free from defects). Compression inredundancy space may occur for either column-repair space or row-repairspace. For example, a 4 Mb SRAM may be compressed into 1024 row-elementsor 144×4 column-elements. The size of the redundancy space (within thememory array) and the choice of column-repair space or row-repair spaceis usually based on various design, architectural, yield and real estate(die size) tradeoffs, all of which are within the knowledge of one ofordinary skill in the art and, thus, will not be further elaborated uponherein.

Wafer level burn-in circuitry 650 may also include burn-in controlcircuitry 610 which may be coupled to nonvolatile elements 602, signaldetection circuitry 604 and memory array 612 for controlling wafer levelburn-in. Burn-in control circuitry 610 may also be configured forprogramming time stamp bits and to facilitate reading of the wafer levelburn-in data stored in the nonvolatile elements 602. Burn-in controlcircuitry 610 may include a state machine or incremental pointer tofacilitate programming of time stamp bits and antifuse registers.

As shown in FIG. 6, memory device 600 may include a memory array 612including redundancy space, not shown separately as it is included inthe memory array 612. Address compression circuitry 606 is incommunication with the memory array 612 and the BISS and BIST circuitry614 for compressing memory array addresses into redundancy space. Thememory array 612 may have redundant rows and/or columns for making arepair by switching in redundant elements as known to one of ordinaryskill in the art. Memory device 600 may further include burn-in controlcircuitry 610 in communication with the memory array 612 for controllingwafer level burn-in and storing wafer level burn-in data in thenonvolatile elements 602. Signal detection circuitry 604 is incommunication with the burn-in control circuitry 610 for detecting asignal indicating a transition between wafer level burn-in stress andself-test modes. As described above, the signal may be a shortsupervoltage pulse on an antifuse supply CMN. Built-in self-stresscircuitry (BISS) and built-in self-test (BIST) circuitry 614 is on diecircuitry that may be used to stress the die at elevated supply voltagesand to detect failures after each cycle of stress which are recorded innonvolatile elements 602, as discussed above.

Wafer level burn-in circuitry 650 of the present invention may be usedin any higher order digital logic device or IC which may be suitable forwafer level burn-in to generate burn-in reliability curves. Memorydevice 600 may be, for example and not by way of limitation, a dynamicrandom access memory (DRAM), double data rate synchronous DRAM (DDRSDRAM), RAMBUS® DRAM (RDRAM®), extended data-out DRAM (EDO DRAM),fast-page-mode DRAM (FPM DRAM), static random access memory (SRAM),SyncBurst™ SRAM, Zero Bus Turnaround™ SRAM (ZBT™ SRAM), Quad Data Rate™SRAM (QDR™ SRAM), double data rate synchronous SRAM (DDR SRAM) andnonvolatile electrically block-erasable programmable read only memory(Flash).

Referring to FIG. 7, a plan view of a bulk semiconductor substrate 700configured as a wafer is shown including a plurality of IC dice, onlyone IC die 702 being shown for clarity. Each IC die 702 may includewafer level burn-in circuitry 650 of the present invention. IC die 702may be a memory device as described above, or it may be any otherintegrated circuit die that includes wafer level burn-in circuitry 650of the present invention. Semiconductor substrate 700 may also be alarge scale bulk substrate comprising a layer of semiconductor material,other than a conventional silicon wafer.

FIG. 8 is a block diagram of a system 800 for performing wafer levelburn-in in accordance with the present invention. System 800 may includea high current VCC/VSS power supply 802 configured for providing power,VCC, and ground, VSS, to a wafer 804 during wafer level burn-in througha sacrificial metal layer on the wafer 804. System 800 may furtherinclude a high current power supply CMN 806 suitable for programmingnonvolatile elements. High current power supply CMN 806 may also be usedto provide a supervoltage that may be detected by supervoltage detectioncircuitry in each IC die. The supervoltage provides a logic signal toswitch between a BISS mode and a BIST mode and for storing failuresdetected in nonvolatile elements (wafer level burn-in data) inaccordance with the present invention. Wafer level burn-in data may bestored in nonvolatile elements (not shown) in each IC die (also notshown) on the wafer 804. Wafer 804 may be bulk semiconductor substrate700 as described above with regard to FIG. 7. System 800 may furtherinclude a wafer probe tester 808 for reading the wafer level burn-indata to generate at least one wafer level burn-in reliability curve inaccordance with the present invention. Wafer probe tester 808 may beused to effect repairs in each of the IC dice on the wafer 804 byprogramming in redundant elements.

The semiconductor technology employed is not a limiting factor in theapplication of the circuits and systems of the present invention. Whilesilicon is the presently preferred bulk semiconductor material forcommercial electronic devices, gallium arsenide and indium phosphidesubstrates may also be employed. Of course, it will be understood thatthe devices of the present invention may be fabricated on othersemiconductor substrates as well, including, for example,silicon-on-glass (SOG) substrates, silicon-on-insulator (SOI)substrates, and silicon-on-sapphire (SOS) substrates.

Although this invention has been described with reference to particularembodiments, the invention is not limited to these describedembodiments. The invention is limited only by the appended claims, whichinclude within their scope all equivalent devices or methods thatoperate according to the principles of the invention as describedherein.

1. A method for storing numbers of failures detected in an integratedcircuit (IC) die on a bulk substrate at discrete time intervals duringwafer level burn-in, comprising: providing nonvolatile elements forstoring wafer level burn-in data in said IC die; providing built-inself-stress (BISS) circuitry in said IC die; providing built-inself-test (BIST) circuitry in said IC die; initially testing said IC dieusing said BIST circuitry; storing a number of failures detected in saidnonvolatile elements; stressing said IC die using said BISS circuitry;testing said IC die using said BIST circuitry; and repeating as statedabove until a predetermined number of cycles of wafer level burn-in arecompleted.
 2. The method of claim 1, further comprising detecting asignal indicating that the act of repeating should be performed.
 3. Themethod of claim 2, wherein detecting the signal comprises detecting asupervoltage on the signal.
 4. The method of claim 1, further comprisingreading the number of failures stored in the nonvolatile elements. 5.The method of claim 1, further comprising storing a time stamp in thenonvolatile elements, wherein the time stamp is associated with thenumber of failures and a cycle of the predetermined number of cycles. 6.The method of claim 5, further comprising reading from the nonvolatileelements the number of failures and the time stamp associated with thenumber of failures.
 7. The method of claim 1, further comprising:switching between the acts of stressing the IC die using the BISScircuitry and testing the IC die using the BIST circuitry; and detectinga switching signal indicating that the act of switching should beperformed.
 8. A method of screening unused antifuse registers duringwafer level burn-in, comprising reading a time stamp bit for eachantifuse register of a plurality of antifuse registers to determinewhether each antifuse register of the plurality has been used or has notbeen used.
 9. The method of claim 8, comprising: performing a built-inself test (BIST) to determine a current number of failures in anintegrated circuit on a wafer, wherein the integrated circuit includesthe plurality of antifuse registers; storing the current number offailures from the BIST in an unused antifuse register of the pluralityof antifuse registers; and storing a time stamp associated with the BISTin the unused antifuse register.
 10. The method of claim 9, furthercomprising repeating the acts of performing a BIST, storing the currentnumber of failures, and storing the time stamp associated with the BIST,for an additional BIST cycle.
 11. The method of claim 9, whereinperforming the BIST is initiated by detecting a signal indicating atransition between a number of cycles of burn-in testing of the wafer.12. The method of claim 11, wherein detecting the signal comprisesdetecting a supervoltage on the signal.
 13. The method of claim 9,further comprising reading from the plurality of antifuse registers thenumber of failures and the time stamp associated with the number offailures.
 14. The method of claim 9, further comprising: performing abuilt-in self stress (BISS) test on the integrated circuit on the wafer;switching between the acts of performing the BIST and performing theBISS; and detecting a switching signal indicating that the act ofswitching should be performed.